When information is missing or inconsistent, the Census Bureau uses a method called imputation to assign values. Imputation relies on the statistical principle of "homogeneity," or the tendency of households within a small geographic area to be similar in most characteristics. For example, the value of "rented" is likely to be imputed for a housing unit not reporting on owner/renter status in a neighborhood with multiunits or apartments where other respondents reported "rented" on the census questionnaire. In past censuses, when the occupancy status or the number of residents was not known for a housing unit, this information was imputed.

An operation which allows respondents to use the Census Bureau's Internet site to (1) ask questions and receive answers about the census form, job opportunities, or the purpose of the census and (2) provide responses to the short form.

Interpolation frequently is used in calculating medians or quartiles based on interval data and in approximating standard errors from tables. Linear interpolation is used to estimate values of a function between two known values. Pareto interpolation is an alternative to linear interpolation. In Pareto interpolation, the median is derived by interpolating between the logarithms of the upper and lower income limits of the median category. It is used by the Census Bureau in calculating median income within intervals wider than $2,500.

The data on private vehicle occupancy were derived from answers to long-form questionnaire Item 23b, which was asked of a sample of the population 15 years old and over. This question was asked of people who indicated in Question 21 that they worked at some time during the reference week and who reported in Question 23a that their means of transportation to work was "Car, truck, or van." (For more information, see "Reference Week.") Data were tabulated for workers 16 years old and over; that is, members of the armed forces and civilians who were at work during the reference week.

Private vehicle occupancy refers to the number of people who usually rode to work in the vehicle during the reference week. The category "Drove alone," includes people who usually drove alone to work as well as people who were driven to work by someone who then drove back home or to a nonwork destination. The category "Carpooled," includes workers who reported that two or more people usually rode to work in the vehicle during the reference week.

This is obtained by dividing the number of people who reported using a car, truck, or van to get to work by the number of such vehicles that they used. The number of vehicles used is derived by counting each person who drove alone as one vehicle, each person who reported being in a 2-person carpool as one-half of a vehicle, each person who reported being in a three-person carpool as one-third of a vehicle, and so on, and then summing all the vehicles. Workers per car, truck, or van is rounded to the nearest hundredth.

The terms "worker" and "work" appear in connection with several subjects: employment status, journey-to-work, class of worker, and work status in 1999. Their meaning varies and, therefore, should be determined by referring to the definition of the subject in which they appear. When used in the concepts "Workers in Family," "Workers in Family in 1999," and "Full-Time, Year-Round Workers," the term "worker" relates to the meaning of work defined for the "Work Status in 1999" subject.